The speed in which tape media storage and retrieval libraries access new tape cassettes is critical in an interactive environment in order to provide rapid response to requests for data. Tapes must be rewound upon the completion of reading and/or writing data so that the tape cassette, upon being stored, will be in the appropriate state for immediate use upon being accessed. Tape libraries are handicapped because of the low rewind tape transport speed.
The problem is that tape cassettes contain very long lengths of tape in order to achieve the maximum capacity. Thus, the rewind length from the last position of the tape at the completion of reading and/or writing back to the starting position, typically called "BOT" for "beginning of tape", is likely to be very long. In two reel cassettes, the BOT is typically at one end of the tape, but may also be at the midpoint to reduce the initial search time.
Tape library media are optimized for lowest cost and greatest capacity in the smallest form factor, for which tape cassettes and cartridges comprises the best design choice. Many tape cassettes and cartridges use a relatively wide tape for high capacity, e.g., half inch tape, and the associated library read/write tape drives have only mechanical tape guides, no air bearings. Thus, the rewind must be accomplished at a low speed in order to avoid damaging the tape edge. Further, in order to keep the library cost low, library tape drives may not have a tape lifter or head retractor so that the tape must run against the head during rewind, requiring that the rewind be accomplished at a very low speed to prevent excessive head wear. Lastly, space within the library for the read/write drive may be limited, and the read/write drive motor may be of limited size and not have sufficient torque to rewind at high speed.
Although the problem is primarily related to tape cassettes, one type of single reel tape cartridge utilizes a single loop of tape, where the tape is fed from the inner radius of the reel and the tape is recovered on the outer radius of the reel. Thus, the rate at which the tape can be rewound is limited additionally by the heat generated by the friction of the tape layers slipping by one another.
All of the tape cassettes and cartridges are optimized for low cost, such as by having low cost bearings, so that the cassette may be the limiting factor for rewind speed.
As the result, tape libraries often add additional read/write drives in order to improve the average response time. This is a costly solution since the read/write drive, particularly the tape head therein, is one of the most expensive components of the library.